There is the general problem in medical imaging methods for imaging the heart and the blood vessels, in particular the pericardiac ones, that because of the heart beats the body region to be recorded is subject to a continuous periodic movement on the basis of which it is possible to compare directly with one another only images that have been recorded at the same point in the cardiac cycle. This circumstance is problematic, particularly in the case of X-ray tomography recording methods in the case of which the image data record (tomogram) to be created is calculated by backprojection of a multiplicity of projection images recorded from a different projection angle. The backprojection manages to be free from interference only when the basic projection images image an identical three-dimensional body region. However, if because of the movement of the heart, for example, the body region moves during the recording of the projection images, this is reflected in movement artifacts in the resulting tomogram, and these can substantially limit the quality of the latter.
It is therefore customary in the case of X-ray tomographic pictures of the heart and/or the pericardiac blood vessels to synchronize the recording of the projection images with the cardiac rhythm of the patient in such a way that the imaging is respectively performed in the rest phase of the cardiac cycle. An appropriate triggering of the imaging is performed in this case with the aid of the ECG (electrocardiogram) signal from the patient. The ECG-assisted control of imaging is also denoted as “ECG gating”. The ECG assisted modulation of the tube current is denoted here as ECG pulsing.
The more so as only the projection images recorded within the rest phases of the cardiac movement can be used to reconstruct the tomogram, it is also only during the time windows corresponding to these rest phases that there is a need to apply X-radiation to the patient. In order to reduce the X-ray dose applied to the patient, it would be desirable, on the other hand to reduce the irradiation as far as possible outside these time windows.
In conventional imaging methods with ECG pulsing, the normal approach is to estimate the time window following in time, that is to say occurring in the direct future, by forming the mean or the median over a prescribed number of preceding cardiac cycles, and to generate a recording pulse, corresponding to the precalculated time window, for modulating the tube current during imaging.
In the event of irregularities in the cardiac rhythm, that is to say a change in the heart rate on the time scale of one or a few cardiac cycles, this method always leads, however, to a more or less pronounced miscalculation of the predicted time window.
In order to ensure that the image information required for image reconstruction is obtained despite the uncertainty in the calculation of the recording pulses, in the case of conventional recording methods the radiation is not completely switched off outside the predicted time windows, but reduced to a specific fraction, for example 25%. It is thereby possible to calculate a tomogram even in the event of a miscalculation of the time window. However, as a rule a miscalculation of the time window is reflected in a substantially restricted quality of the tomogram, in particular in increased image noise.
Because of these restrictions, in clinical practice ECG pulsing is normally used only with patients who have a very uniform cardiac rhythm. In many instances, by contrast, ECG pulsing is switched off during imaging, and so the patient experiences a full dose of irradiation during the entire scan, and a substantially greater dose than required is therefore applied.